Can the spin-delocalization effect express itself at the transition state of the hydrogen-atom abstraction reaction? Correlation analysis of relative rates measured by a rigorous methodology for eleven p-Y-substituted isopropylbenzenes

Abstract

A rigorous procedure has been developed for measuring the relative rates, i.e., k r(Y) = k Y/ k H, of hydrogen-abstraction from eleven substituted iso-propylbenzenes ( 1-Y, with Y = NO 2, CN, F, Cl, Br, CO 2Me, Me, tBu, COMe, OMe and SMe) by bromine atoms. The reaction was run in cyclohexane in the presence of HBr, O 2 and dibutyl peroxyoxalate at 5O°C. It has been demonstrated that the H-atom on step shown by is the measured step, and all products are derived from the intermediate benzylic radical 4. Correlation analysis of these rate data seems to suggest that a spin effect is also operating at the transition state of H-atom abstraction reaction, even though the polar effect predominates. Among various combinations of σ x and σ . for the dual-parameter, the (σ p+σ . jj) combination yields the best correlation.